1. Field of the Invention
This invention relates to the field of semiconductor memories as fabricated in integrated circuits and more particularly to the antifuse structure used within a read-only memory and a method for reducing the electrical resistivity of the antifuse in its programmed state.
2. Description of the Prior Art
An antifuse is defined herein as a substantially nonconductive structure that is capable of becoming substantially conductive upon application of a certain voltage.
Examples of the antifuse and it use as a memory cell within a Read-Only Memory are discussed in Roesner, "Method of Fabricating a High Density Programmable Read-Only Memory", U.S. Pat. No. 4,796,074 (1989) and Roesner, "Electrically Programmable Read-Only Memory Stacked above a Semiconductor Substrate", U.S. Pat. No. 4,442,507 (1984), each of which references are herein expressly incorporated by reference.
A typical antifuse structure is formed by disposing a layer of heavily N-doped word line on an insulating substrate, disposing an antifuse layer of lightly N-doped programmable material within an etched cavity in an overlying oxide layer, and disposing a metallic address line upon and connected to the antifuse layer.
The programmable material used for the antifuse layer might be selected from the group of silicon, germanium, carbon and alpha-tin. The properties of the programmable material are such that the material exhibits a relatively high resistance as long as the voltage across it does not exceed a threshold level. However, once the threshold voltage is exceeded, the resistance of material irreversibly switches form a high resistance state to a relatively low resistance state.
Antifuse structures as described above and in previous patents '507 and '074 operate by forming a conductive filament in an initially nonconductive film. The antifuse has a very high resistance initially for two reasons. First, the film is amorphous, resulting in very low electron mobility. Second, any conductive elements or dopants which exist in the film have not been activated and therefore do not enhance the conduction of carriers in the film.
To program or change the resistance of the antifuse from a very high level (greater than 100,000,000 ohms) to a low level (less than 1000 ohms), a voltage of sufficiently high electrical field strength is placed across the antifuse film to cause dielectric breakdown. As breakdown occurs electrical current will flow through one small region of the film. The current is limited by the resistance of the filament itself as well as any series resistance of conductive layers or logic devices (transistors) in series with the antifuse. This flow of current in a very small area converts the region from amorphous to single crystalline material and at the same time activates any residual carriers.
It is desirable to reduce the resistance of the programmed antifuse to as low a level as possible since the higher resistance is more restrictive to electrical signals passing through it. In other words, higher resistance connections cause circuits to operate at slower speeds.
The resistance of a programmed antifuse can be reduced by either increasing the actual size of the antifuse filament or decreasing the resistivity of the filament material itself. In order to increase the size of the filament, more current during programming is required. This actually increases the amount of energy released in the filament which causes a larger area to be altered to single crystalline. Such a technique is presently being used to reduce the resistance in those applications where the lower resistance is critical.
The electrical characteristics of an antifuse prior to programming are typically not identical when measured in the forward and reverse directions. In this context the forward direction is the direction that current flows when a positive voltage is applied to the top node of the antifuse, e.g. the metallization, while the opposing end of the antifuse or bottom conductor is grounded. The reverse direction is the direction of current flow when a negative voltage is applied to the top node and the opposing end of the antifuse or bottom conductor is grounded. For example, if the forward programming voltage is 10 volts, the reverse programming voltage will typically be 12 volts. Asymmetrical programming voltages can be undesirable is some applications. The higher reverse voltage must be accommodated for the circuit design, which can be a burdensome design requirement to implement. A lower forward programming voltage places an upper limit on the operational voltage of the circuit if inadvertent programming of the antifuses are to be avoided.
Accordingly, what is needed is a method of decreasing the resistivity of the filament material in the antifuse in its programmed state. What is also needed is a method of obtaining the decreased resistivity of the programmed antifuse without also significantly decreasing the resistivity of the antifuse in its initial state prior to being programmed. What is further needed is a compatible methodology and structure for making the forward and reverse programming voltages symmetrical.